1. Field of the Invention
This invention relates to chromatographic separation and more particularly relates to an apparatus system and method for chromatography using a water stationary phase.
2. Background Information
Water offers an appealing alternative to form one or both the mobile and stationary phase of a chromatographic separation system as it is inexpensive, safe and environmentally friendly. Since the advent of column chromatography, water has been employed as the mobile phase, stationary phase, or components thereof, for various modes of analytical separations. For example, water has been used for years as a main mobile phase component in high performance liquid chromatography (HPLC), and has commonly been combined with a variety of organic solvents and inorganic buffers. Water has also been incorporated into the mobile phase in packed column (Nonaka, 1972) and capillary column (Berezkin et al., 1975) steam chromatography as well as in packed column (Geiser et al., 1988) and capillary column (Schwartz and Fresenius, 1988) supercritical fluid chromatography (SFC). In recent years however, pure water has perhaps found its niche as a mobile phase in subcritical water chromatography (SWC) which has evolved from its initial description (Guillemin et al., 1981; Smith and Burgess, 1996; Miller and Hawthorne, 1997) into a fairly widely used technique (Yang, 2007; Smith, 2008; Hartonen and Riekkola, 2008). This stems from water's advantageous qualities over organic solvents such as compatibility with flame ionization detection (FID) and thermally tuneable polarity (Akerlof and Oshry, 1950). In order to take advantage of this temperature-polarity relationship, high temperatures are required to emulate the polarities of conventional, reversed phase HPLC solvents. The maximum temperature (lowest polarity) attainable is therefore limited by the thermal stability of the stationary phase (Yang, 2006).
The incorporation of water as a stationary phase component has been explored and utilized to a much lesser extent. For example, there have been a few reports of water forming the stationary phase on solid support in the early stages of the development of gas chromatography (GC) (Purnel and Spencer, 1955; Phifer and Plummer, 1966; Karger and Hartkoph, 1968; Karger, 1969). However, water's utility as a GC stationary phase was short lived due to its very polar nature and high volatility. While these limitations precluded the use of water as an analytical stationary phase, it has also been used chromatographically in physical chemistry to quantify various solution (Pescar and Martin, 1966; Shaffer and Daubert, 1969) and interfacial (Karger et al., 1971; Hartkoph and Karger, 1970) properties of single compounds. Also of interest, aqueous solutions containing various ions have been used as GC stationary phases in order to alter analyte selectivity using conventional (King, 1975) and water-saturated (Berezkina et al., 1966) carrier gases. In addition to its utility in GC, there have also been reports of its use as a HPLC stationary phase, both coated as a liquid phase on a solid support (Martin and Synge, 1941) and as a solid, water-ice phase (Dasgupta and Mo, 1997). Alternatively, liquid water has formed a pseudo-stationary phase (or component thereof) used in the absence of solid support particles in a number of alternative separation techniques such as counter-current chromatography (CCC) (Foucault, 1991) and co-current chromatography (Lucy and Hasuermann, 1995).
Other related biphasic separation systems employing gas/liquid systems (Wells et al., 2002; Perilloux and Deans, 1972) have also been reported (Fogwill and Thurbide, 2007; Fogwill and Thurbide, 2008). In addition, systems employing supercritical CO2 have been recently described. For example, supercritical fluid CCC successfully separated acetophenone and benzophenone operating at 435 rpm in 3 hours (Yu et al., 1996). More recently, methanol/supercritical CO2 systems have been explored by the Parcher group employing a slowly moving methanol pseudo-stationary phase formed along the wall of an uncoated capillary (Luo et al., 2003; Wang et al., 2006). In this instance, the successful separation of light, n-alkanes was demonstrated.